Many aircraft designs are optimized for one specific flight mode, while those designs may have reduced efficiency in other flight modes or conditions. For example, some aircraft are more efficient at lower speeds than at higher speeds, and some aircraft are more efficient at higher altitudes than at lower altitudes.
In one example, motors and propellers are commonly tuned to match the specific performance required for a given flight condition, and deviation from such tuning or from the intended flight condition often results in a reduction in efficiency. For example, in a vertical take-off and landing aircraft (such as a tailsitter aircraft), a high-power motor and propeller may be used to provide vertical lift, while that same high-power motor and propeller may be inefficient during horizontal cruise operations. Conversely, a low-power motor and propeller may be efficient for horizontal cruise operations, while that same low-power motor and propeller may not provide sufficient thrust for vertical lift. Accordingly, an aircraft running one type of propeller for both vertical lift and horizontal cruise may be inefficient in either or both flight conditions. Similarly, an aircraft in horizontal cruise may have a propeller and motor system that is efficient in one horizontal flight condition but not in other horizontal flight conditions.
Some aircraft may include two different propulsion systems, such as one for lift and one for horizontal thrust, or one for low-speed horizontal cruise and one for high-speed horizontal cruise. But one of these propulsion systems would generally be unused while the other was operating, such that the aircraft would effectively be carrying unnecessary weight.